How do Priests know when a soul escapes from purgatory?

The Catholic Answers

Souls do not escape from purgatory as criminals from jail. When they are sufficiently purified for the Vision of God they are admitted to heaven. And no one knows when this occurs, unless God gives a special revelation, a favor we have no right to ask.

Radio Replies Volume 1 by Rev. Dr. Leslie Rumble MSC and Rev. Charles Mortimer Carty